Improve cache download behvaiour

[pfitzseb]

Currently, we're trying to download cache files for all packages even if they all time out, which can take quite a bit of time. Would be good to add a heuristic that aborts after the first N downloads time out (or otherwise fail).

[carstenbauer]

Just ran into this so +1.

Actually, I also wonder why all the download attempts time out. On the system, I use a proxy to access the internet but installing Julia packages and such works just fine.

[xgdgsc]

It seems the download doesn' t use proxy settings from vscode "http.proxy"? Can the cache be hosted at pkg.julialang.org to reuse the existing mirroring framework? Or provide a one liner cli command that use proxy setting from shell if failed downloading?